Alarm circuit

ABSTRACT

An alarm circuit for vehicle headlights including a switching circuit which provides a warning when the headlights should be activated in accordance with ambient conditions, such as low light levels, as well as warning of when the headlights should be de-activated, as when the vehicle ignition system is switched off.

This invention relates to an alarm circuit for indicating changes in ambient conditions--such as light level--in accordance with the selected operating mode of a machine relative to such conditions.

A particular environment to which the present invention is especially suited is that of the automobile. In an automobile, various accessories may be selectively operated in accordance with prevailing conditions--such as windshield wipers for rain or snow, headlights for low light levels, etc. Many of these accessories are only operable when the automobile ignition is switched on, but others do not automatically turn off when the ignition is turned off, such as, for example, the headlights. It is, perhaps, the most common single cause of battery failures in automobiles that the headlights are inadvertently left burning when the vehicle is parked, thus imposing a severe drain on the battery. Another problem is that many drivers find the ambient light level at dusk or dawn difficult to judge and, therefore, do not energize the vehicle headlights at the proper time. Statistically, many accidents occur during these periods as a result of the poor light conditions and failure of the driver or drivers involved to use their vehicle headlights. Another common problem is that of a driver entering a car at night and forgetting to switch on the headlights before operating the vehicle.

It is therefore an object of the present invention to provide an alarm circuit which will provide warning of when a particular operating function--such as vehicle headlights--should be activated in accordance with ambient conditions, as well as providing warning of when such function should be de-activated due to a change in other operating conditions--such as switching off of the vehicle ignition system.

Thus, according to the present invention there is provided an alarm circuit comprising a first terminal means adapted for connection to a supply potential source, an alarm means capable of being energized by said supply potential and connected between said first terminal means and a reference ground potential point through first and second series-connected switching means; third and fourth series-connected switching means connected between said first terminal means and a reference ground potential point; each of said first, second, third and fourth switching means having a control electrode for selectively enabling or disabling said switching means in accordance with the potential at the respective control electrode; said control electrode of said first switching means further being connected through a series-connected first unidirectional current passing means and a sensing circuit to second terminal means and, the control electrode of said first switching means being connected through second unidirectional current passing means to third terminal means, whereby said first switching means closes in response to potential from said third terminal means and said sensing circuit, said first and second unidirectional current passing means isolating said second and third terminal means from one another whilst providing current paths therethrough from said second and third terminal means respectively to said control electrode of said first switching means, and the control electrodes of said second, third and fourth switching means being connected to said first, third and second terminal means, respectively, whereby said third and fourth switching means close in response to potential from both of said second and third terminal means and thereby cause said second switching means to open, and said second switching means is closed when at least one of said third and fourth switching means is open.

In a preferred embodiment of the invention, each of switching means is a transistor, which is of the appropriate polarity type, depending upon the polarity of the supply potential for the circuit.

As hereinbefore stated, the circuit is of special use as an automobile headlight warning system, wherein said second and third terminal means are respectively connected to the output side of the automobile ignition and headlight switches. Said first terminal means is connected to the ignition system of the automobile, so that when ignition switch is ON and the headlights switch is OFF, the alarm would be energized if the ambient light, as detected by the sensing circuit, is below a certain level. Also, if the headlights switch is ON and the ignition switch is turned OFF, the alarm will again be energized.

The invention will be more readily understood from the following description of an embodiment thereof given by way of example only and with reference to the accompanying drawings, wherein:

FIG. 1 is a block-diagram of an alarm circuit; and

FIG. 2 is a schematic circuit diagram of one embodiment of the alarm circuit illustrated in FIG. 1.

Referring to FIG. 1, the novel circuit comprises first and second switching means Q₁ and Q₂ serially connected with an alarm between a voltage supply terminal V₁ and a reference ground potential point. Each of switches Q₁ and Q₂ has a control element which causes the switch to conduct upon the application of a suitable bias potential thereto. The control element for switching means Q₁ is connected through a sensor circuit and a unidirectional current passing means D₂ to a second voltage supply terminal V₂, and is also connected to a third voltage supply terminal V₃ through a unidirectional current passing means D₁.

The control element for switching means Q₂ is connected to voltage supply terminal V₁ and is also connected to reference ground potential through serially connected switching means Q₃ and Q₄, each of which is also provided with a control element. The control element of Q₃ is connected to terminal V₃ and the control element of Q₄ is connected to terminal V₂.

The circuit operation is as follows. For the alarm to be energized, there must be a current path through the alarm and through switches Q₁ and Q₂, between voltage supply terminal V₁ and reference ground potential. Therefore, both switches Q₁ and Q₂ require a bias potential appearing at the control elements thereof to maintain the switches in conducting state. Considering firstly the conditions under which switch means Q₂ is conducting, the necessary bias voltage will be derived from terminal V₁, unless current is bled to ground through Q₃ and Q₄. For the latter situation to occur, both Q₃ and Q₄ must be conducting, which requires a bias voltage to be applied to each switch from the respective terminals V₃ and V₂. In this case, current would flow through Q₃ and Q₄, and Q₂ would not conduct, thereby preventing energization of the alarm.

Considering the necessary conditions for Q₁ to conduct, the required bias potential may be derived either from terminal V₃ (directly) or from terminal V₂ (through the sensor circuit). If a bias potential appears at V₃, then Q₁ is caused to conduct, regardless of the condition of the sensor circuit or the potential of terminal V₂. If no bias potential appears at V₃, then Q₁ can only conduct if a bias potential is derived from terminal V₂, through the sensor circuit.

The foregoing may thus be summarized as follows:

If both V₂ and V₃ are at bias potential, both Q₃ and Q₄ will conduct, which causes Q₂ to block. The alarm is then "off".

If neither V₂ and V₃ are at bias potential, Q₂ is caused to conduct since the control element thereof is at the potential source for Q₁ and, thus, the alarm is again "off".

If only V₃ is at bias potential, then Q₁ and Q₃ conduct. Q₄ remains non-conducting since V₂ is not at bias potential, and Q₂ is therefore biased into conductance by the potential of V₁. Thus, both Q₁ and Q₂ conduct and the alarm is energized.

If only V₂ is at bias potential, then Q₃ remains non-conducting, which permits the bias potential of V₁ to appear at the control element of Q₂. If the conditions are appropriate for the sensor circuit to be completed, then the bias potential of V₂ is applied to the control element of Q₁. It may be noted that diode D₁ prevents the control element of Q₃ or the terminal V₃ from assuming the potential of terminal V₂. Therefore, both Q₁ and Q₂ conduct, which energizes the alarm. However, if the conditions are such that the sensor circuit is not complete, then the bias potential of V₂ is not applied to the control element of Q₁, and Q₁ remains non-conducting. In this condition, the alarm is "off".

Consider the situation where V₂ and V₃ are interconnected with the ignition and light switches, respectively, of an automobile, and the sensor circuit is completed upon detection of ambient light level below a certain predetermined value. Thus, when the ignition switch is "on" and the light switch is "off", a bias potential appears at V₂, but not at V₃. It will now be apparent from a consideration of the foregoing discussion that in such a situation, the potential of V₂ (i.e. the ignition switch) appears at the control element of Q₁ only when the ambient light level drops below the predetermined level for which the sensor is set, and the alarm is then energized.

Conversely, when the light switch is "on" and the ignition switch is "off", Q₁ is caused to conduct, regardless of the condition of the sensor circuit, thus energizing the alarm and giving warning that the lights have been left on after the ignition has been switched off.

Referring now to FIG. 2, a preferred embodiment of the invention is illustrated, wherein switching means Q₁ to Q₄ inclusive are bipolar transistors (now designated T₁ to T₄, respectively). Since the electrical systems of almost all modern automobiles are negative ground, NPN transistors are employed throughout. Again, terminal V₁ is maintained at a steady potential (normally 12 volts in a modern automobile) and terminals V₂ and V₃ are connected to the switched sides of the ignition and headlight switches respectively.

The sensor circuit comprises a capacitor C₁ connected through a zener diode ZD₁ to the gate of a silicon controlled rectifier SCR₁. In this case, SCR₁ also constitutes the unidirectional current passing means D₂ of FIG. 1. A photo-resistor R₂ is connected across capacitor C₁, and the negative plate of C₁ is connected to ground. The gate of SCR₁ is also connected to ground through a resistor R₃. The positive plate of capacitor C₁ is connected through a variable resistor VR₁ to the anode of SCR₁, the junction between SCR₁ and VR₁ being connected to terminal V₂.

The cathode of SCR₁ is connected through a diode D₁ to terminal V₃ and is also connected through a resistor R₄ to the base of transistor T₁.

The base of transistor T₂ is connected to the collector of transistor T₃ through a resistor R₅, and the base of T₃ is connected through a resistor R₆ to the junction of diode D₁ and terminal V₃. The base of T₃ is also connected through a resistor R₇ to ground.

The base of transistor T₄ is connected through a resistor R₈ to terminal V₂ and is also connected through a resistor R₉ to ground. The emitter of T₄ is connected directly to ground.

A diode D₃ is connected between the terminal V₁ and the alarm, such diode protecting the circuit against polarity reversal of the operating potential at V₁. The cathode of diode D₃ is connected through a resistor R₁₀ to the collector of T₃.

The alarm is connected to the collector of T₁ ; the emitter of T₁ is connected to the collector of T₂ ; the emitter of T₃ is connected to the collector of T₄ ; and the emitter of T₂ is connected to ground.

The operation of the circuit will now be described with reference to the following conditions:

Ignition OFF, Headlights ON

Transistor T₁ is enabled by the bias potential derived through D₁ and R₄ from terminal V₃. Transistor T₃ is also enabled by the bias potential derived from terminal V₃ through resistors R₆ and R₇. The potential at V₃ is, of course, the 12 volts supply for the headlight system.

Transistor T₄ is disabled, since there is no bias potential supply from terminal V₂, and transistor T₂ is therefore enabled by a bias potential derived from terminal V₁ through R₁₀ and R₅. With both T₁ and T₂ enabled, the current path through the alarm to ground is completed and the alarm is energized.

As soon as the headlights are turned OFF, the bias potential at terminal V₃ is removed, thus disabling transistor T₁ and interrupting the current path from the alarm to ground. Therefore, the alarm is de-energized.

Ignition ON, Headlights OFF

Terminal V₂ is now at the 12-volt supply potential and T₄ is therefore enabled. With the headlight switch in the OFF position, there is no bias potential at V₃, and T₃ is consequently disabled. Therefore, there is no potential drop across R₁₀ and T₂ is enabled through R₁₀ and R₅.

The condition of the alarm is now entirely dependent upon the state of transistor T₁. Provided the ambient light level is above a predetermined value (which is set by adjustment of variable resistor VR₁), the resistance of R₂ is sufficiently low that the voltage appearing at the cathode of zener diode ZD₁ is below the breakdown voltage of ZD₁. There is no voltage drop across resistor R₃ to ground and, consequently, no enabling potential at the gate of SCR₁. Thus, SCR₁ does not conduct, and transistor T₁ remains disabled.

If, however, the ambient light level drops below the predetermined value, the resistance of R₂ increases to a point where the voltage appearing at the cathode of ZD₁ exceeds the breakdown voltage thereof and ZD₁ conducts. A voltage drop now occurs across R₃ and an enabling potential appears at the gate of SCR₁. Since SCR₁ is now conducting, an enabling potential appears at the base of T₁ through SCR₁ and resistor R₄, and both T₁ and T₂ are enabled. In this condition, the alarm is energized.

In the foregoing discussion, the role of capacitor C₁ has been ignored for the sake of simplicity. The function of C₁ is to introduce a time factor into the sensor circuit, whereby the alarm will not be energized by transient drops in light level such as may be caused by overpasses and the like. Thus, as the light level drops and the resistance of R₂ increases, capacitor C₁ charges correspondingly. Providing the light level remains low, sufficient charge will be developed on capacitor C₁ to effect a breakdown of ZD₁ and consequent enabling of SCR₁. However, if the light level drops are transitory, the charge developed on C₁ will be insufficient to provide a breakdown voltage for ZD₁ and SCR₁ remains disabled.

The component values and transistor types, etc. are chosen in accordance with the precise operating parameters of the circuit, including the operating voltage level and polarity. The transistors illustrated are bipolar NPN types, but in the case of positive-ground systems would be PNP types. Also, by appropriate circuit modifications, which may be readily determined by those skilled in the art, the bipolar transistors may be replaced by field-effect transistors or, indeed, any other suitable type of switching means.

The alarm may be a bell, a visual warning, a buzzer, or a combination of warning devices. The sensor portion of the device is by no means limited to a light-level sensor circuit, as exemplified herein, but may be responsive to other conditions of which it is desired to provide warning.

Thus, it is believed to be apparent that the present invention has a wide variety of applications and may exist in many alternative embodiments to those described above without departing from the spirit and scope of the invention as described and claimed herein. 

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:
 1. An alarm circuit comprising a first terminal means adapted for connection to a supply potential source, an alarm means capable of being energized by said supply potential and connected between said first terminal means and a reference ground potential point through first and second series-connected switching means; third and fourth series-connected switching means connected between said first terminal means and a reference ground potential point; each of said first, second, third and fourth switching means having a control electrode for selectively enabling or disabling said switching means in accordance with the potential at the respective control electrode; said control electrode of said first switching means further being connected through a series-connected first unidirectional current passing means and a sensing circuit to second terminal means and, the control electrode of said first switching means being connected through second unidirectional current passing means to third terminal means, whereby said first switching means closes in response to potential from said third terminal means and said sensing circuit, said first and second unidirectional current passing means isolating said second and third terminal means from one another whilst providing current paths therethrough from said second and third terminal means respectively to said control electrode of said first switching means; and the control electrodes of said second, third and fourth switching means being connected to said first, third and second terminal means, respectively, whereby said third and fourth switching means close in response to potential from both of said second and third terminal means and thereby cause said second switching means to open, and said second switching means is closed when at least one of said third and fourth switching means is open.
 2. An alarm circuit as claimed in claim 1, wherein said first, second, third and fourth switching means each comprise first, second, third and fourth transistors, respectively, having base, collector and emitter, and said control electrodes comprise the bases of said respective transistors, said first transistor having its collector connected through said alarm means to said first terminal means, and its emitter connected to the collector of said second transistor, the latter having its emitter grounded, the base of said second transistor being connected to the collector of said third transistor, and the emitter of said third transistor being connected to the collector of said fourth transistor, the emitter of which is grounded.
 3. An alarm circuit as claimed in claim 2, wherein the bases of said third and fourth transistors are returned to ground through biasing resistors.
 4. An alarm circuit as claimed in claim 1, wherein said sensing circuit comprises a photosensitive element controlling the application of potential from said second terminal means to said control electrode of said first switching means.
 5. An alarm circuit as claimed in claim 1, wherein said sensing circuit comprises a photoresistor controlling the application of potential from said second terminal means to said control electrode of said first switching means. 